Chemi-mechanical polishing of semiconductor wafers is useful, at various stages of device fabrication, for planarizing irregular top surface topography, inter alia. For example, in the process of fabricating modern semiconductor integrated circuits, it is necessary to form conductive lines or other structures above previously formed structures. However, prior structure formation often leaves the top surface topography of the silicon wafer highly irregular, with bumps, areas of unequal elevation, troughs, trenches and/or other surface irregularities. As a result of these irregularities, deposition of subsequent layers of materials could easily result in incomplete coverage, breaks in the deposited material, voids, etc., if subsequent layers were deposited directly over the aforementioned highly irregular surfaces. If the irregularities are not alleviated at each major processing step, the top surface topography of the surface irregularities will tend to become even more irregular, causing further problems as layers stack up in further processing of the semiconductor structure.
Depending upon the type of materials used and their intended purposes, numerous undesirable characteristics are produced when these deposition irregularities occur. Incomplete coverage of an insulating oxide layer can lead to short circuits between metallization layers. Voids can trap air or processing gases, either contaminating further processing steps or simply lowering overall device reliability. Sharp points on conductors can result in unusual, undesirable field effects. In general, processing high density circuits over highly irregular structures can lead to very poor yield and/or device performance.
Consequently, it is desirable to effect some type of planarization, or flattening, of integrated circuit structures in order to facilitate the processing of multi-layer integrated circuits and to improve their yield, performance, and reliability. In fact, all of today's high-density integrated circuit fabrication techniques make use of some method of forming planarized structures at critical points in the fabrication process.
Planarization techniques generally fall into one of several categories: chemical/mechanical ("chemi-mechanical", or "chem-mech") polishing techniques; leveling with a filler material then etching back in a controlled environment; and various reflow techniques. Etching techniques can include wet etching, dry or plasma etching, electro-polishing, and ion milling, among others. A few less common planarization techniques exist, such as direct deposition of material into a trench by condensing material from a gaseous phase in the presence of laser light. Most of the differences between modern planarization techniques exist in the points in processing that the different techniques are applied, and in which methods and materials are used.
The present invention is directed to a chemi-mechanical polishing process, which generally involves rubbing a wafer with a polishing pad in a slurry containing both an abrasive and chemicals. Typical slurry chemistry is KOH (Potassium Hydroxide), having a pH of about 11. Generally, polishing slurry is expensive, and cannot be recovered or reused. Typical usage (feed) rates for slurry are on the order of 175 ml (milli-liters) per minute. A typical silica-based slurry is "SC-1" available from Cabot Industries. Another, more expensive slurry based on silica and cerium (oxide) is Rodel "WS-2000".
Chemi-mechanical polishing is described in U.S. Pat. Nos. 4,671,851, 4,910,155, 4,944,836, all of which patents are incorporated by reference herein. When chemi-mechanical polishing is referred to hereinafter, it should be understood to be performed with a suitable slurry, such as Cabot SC-1.
The current state of the art in dielectric film polishing for silicon wafers strongly suggests (requires) the use of more than one polishing pad. For example, two pads are assembled together into a "stack", which may be termed a "composite polishing pad". The (upper) top pad, which performs the actual polishing (i.e., contacts the wafer), is typically stiffer than the more compliant bottom pad, which is mounted to a rotating platen. A pressure sensitive adhesive (PSA) provided by the pad manufacturer on the back face of the pads is typically used to adhere the pads together and to the platen. The platen (and pads) rotates, and a semiconductor wafer mounted to a carrier is lightly urged against the exposed face of the upper polishing pad. The wafer may also be rotated, and may also be moved in an arcuate path across the face of the upper pad, resulting in a complex polishing motion. Generally, the wafer contacts the pad in an area away from its outer edge and away from its center or, in other words, generally in an area described by about the middle two-thirds the radius "r" of the pad (i.e., avoiding the innermost and outermost pad areas corresponding to r=0 through r/6, and 5r/6 through r, respectively).
FIG. 1 shows a typical technique for chemi-mechanical polishing. A first, lower disc-shaped pad 102 (PAD A) having a layer of pressure sensitive adhesive 104 on its back face is adhered (shown exploded) to the front face of a rotating platen 106 (PLATEN). A second, upper disc-shaped pad 108 (PAD B) having a layer of pressure sensitive adhesive 110 on its back face is adhered (shown exploded) to the front face of the lower pad 102. The pads 102 and 108 form a composite pad "stack". The platen 106 is rotated, and a metered stream of slurry 112 (shown as dots) from a slurry supply 114 is delivered via a slurry feed 116 to the front face of the upper pad 108 (PAD B).
Evidently, centrifugal forces and gravity will cause the slurry to flow (wash) over the periphery of the upper pad 108 (PAD B). This is especially evident since the front face of PAD B is specifically intended to be extremely planar for planar polishing of the wafer. This planarity, however, allows the slurry to flow easily off of the face of PAD B, thereby reducing the slurry's "residence time" on the face of the upper pad. Normally, slurry washes over the face of the upper pad before it is efficiently employed (i.e., fully depleted) for polishing, and typically cannot be recovered and recycled.
A silicon wafer 120 is mounted to a carrier 122, and is lightly pressed (flat, coplanar) against the front surface of the upper pad (PAD B) so that formations (on the pad-confronting face of the wafer) sought to be polished are acted upon by the action of the upper pad (PAD B) and the slurry. Typically, the pads 102 and 108 and the platen 106 are on the order of 20-30 inches in diameter, the wafer is 4-6 inches in diameter and, as mentioned above, polishing is performed in the center 2/3 portion of the upper pad (PAD B).
A reservoir 130 contains the platen, pads, carrier, wafer and polishing slurry.
As the slurry is used, it exits the front surface of the upper pad 108 (PAD B) and, as noted above, is not recovered. Evidently, the slurry must be fed onto PAD B at the rate that it exits PAD B, and preferably the rate would be optimized so that the slurry is entirely used, with no loss of un-depleted (un-consumed) slurry. However, this is generally not the case, and the slurry feed rate is established to be higher than would be necessary to deplete the slurry. The slurry is illustrated in FIG. 1 as a single layer of dots on the face of the upper pad, indicating that it is difficult to retain slurry on the front face of the upper pad (PAD B).
Typical pad materials are: (1) for the lower pad 102 (PAD A), foamed polyurethane; and (2) for the upper pad 108 (PAD B), polyester felt stiffened with a polyurethane resin (matrix). The adhesive backings 104 and 110 for the pads are typically polyurethane-based, pressure sensitive adhesive (PSA). Generally, it is preferable that the upper pad (PAD B) is stiffer than the lower pad (PAD A). In the case that both pads are doped with polyurethane resin, this can be achieved simply by doping PAD B with more polyurethane than PAD A. Typical pad thickness is on the order of 0.050 inches.
Two failure modes are of particular interest. In one mode, the slurry is gradually "wicked" into the pads and gradually attacks the adhesive, and adhesive failure can be expected in about three days of usage, which is generally acceptable. This is illustrated at 124, where slurry 112 is shown permeating PAD B and attacking adhesive 110 at the pad-to-pad interface of the upper and lower pads. In another, much more catastrophic mode, the slurry attacks the adhesive bond between PAD A and PAD B directly, edgewise at the adhesive boundary (pad-to-pad interface) between the pads, and failure may occur within one half hour, which is very unacceptable. This is illustrated at 126, where slurry is shown attacking the adhesive 110 edgewise between the two pads. Eventually, and usually abruptly, the pads will delaminate (come unglued) from one another, which will require stopping the polishing process (possibly damaging an in-process wafer), and resetting up the polishing equipment. This is not desirable.
It is known to provide a plastic or rubber ring (dam) around the periphery of the pads and platen, extending upward above the front face of the upper pad (PAD B), primarily for the purpose of creating a reservoir of slurry on the front face of PAD B, which will allow the slurry to be retained longer on the face of PAD B.
FIG. 2 illustrates a technique 200 whereby a rubber ring/dam 202 encircling an upper pad 204, a lower pad 206 and a platen 208, and extending above the front surface of the upper pad 202. In this manner, a "pool" 210 of slurry (shown as several rows of dots) is created on the face of the upper pad, which allows greater residence time in which the slurry can perform its intended function before washing over the top edge of the dam 202.
Use of the dam/ring is primarily directed to optimizing interaction of the slurry with the wafer before the slurry overflows the dam/ring, and requires additional set-up of the polishing apparatus. And, to a much lesser degree, use of the ring/dam may slightly impede slurry from directly attacking the adhesive 212 in the pad-to-pad interface.
Therefore, there remains a need for effectively addressing the problem of catastrophic delamination of the polishing pads due to slurry attacking the adhesive at the pad-to-pad interface.
As noted above, pad replacement is required at regular intervals. A manufacturer of equipment and pads for performing chemi-mechanical polishing is Westech (Phoenix, Ariz.). Pages 13 and 14 of Westech Polisher manual describe the usual process of replacing the pads. As noted therein, "polish pads must be replaced after 15 to 20 hours of operation, or more frequently if there is excessive buildup of [slurry]." Experience shows that replacement may be required after only a few hours of use due to the delamination of the pads resulting from encroachment of the slurry solution into the pad-to-pad interface. It is suspected that the slurry solution acts as a release agent for the PSA (adhesive) film between the pads (see, e.g., FIG. 1, 110).
Understandably, the replacement pads are shipped oversize, larger than the platen to which they will be mounted, so that they may be mounted (adhered) to the platen and then trimmed to size. For purposes of this discussion, the platen has a diameter of 30 inches, and the pads are shipped with a diameter of 33 inches. If the pads were shipped the same size as the platen, a minor misalignment when adhering the pads to the platen would result in exposing the platen surface. This is undesirable for many reasons, among which is the possibility that the slurry will etch the front face of the platen. By way of analogy, those familiar with installing formica (TM) will appreciate that an oversize sheet of formica is fitted to an underlying substrate (e.g., a counter top), and is then trimmed in place to match the peripheral contour of the counter top.
Whether using one or two pads (a two pad system is shown in FIG. 1), the straightforward, recommended technique for trimming the pads is simply to trim them flush around the edge (periphery) of the platen. (See Westech manual, page 14, paragraph "7.") This trimming is suitably accomplished, by hand, with a sharp razor blade or utility knife.
FIGS. 3A and 3B illustrate a technique 300 for assembling polishing pads to a platen for chemi-mechanical polishing. Two pads, an upper pad 302 and a lower pad 304, mounted to a platen 306 (the platen is not visible in FIG. 3A, and the adhesive is omitted for illustrative clarity). In this illustration, neither of the pads are trimmed. As is evident from the illustration, the top pad 302 is slightly mis-aligned over the bottom pad 304. This results in the bottom pad 304 protruding beyond the edge of the upper pad 302 in a circumferential region 308 (shown extending about 180.degree.). Evidently, the protruding region of the bottom pad 304 forms a "shelf" 310 where slurry can readily collect and attack the adhesive film (not shown) between the two pads by migrating between the two pads. This configuration of pads is typical, albeit exaggerated for illustrative purposes. Although polishing can be performed with pads misaligned as shown, it is expected that they would need replacement on a frequent basis.
FIGS. 4A and 4B show the recommended technique 400 for assembling polishing pads to a platen for chemi-mechanical polishing. Two pads, an upper pad 402 and a lower pad 404, are mounted to a platen 406 (lower pad 404 and platen 406 not visible in FIG. 4A; adhesive omitted for clarity). In this illustration, both pads are trimmed according to standard procedure, i.e., to be coincident with the edge of the platen. In this example, as liquid (slurry) runs over the edge of the pad stack, it merely contacts the edge of the interface between the two pads (See FIG. 1). Nevertheless, the slurry can still migrate between the upper and lower pads, attacking the adhesive layer therebetween, and causing premature delamination of the pad stack. Again, although polishing can be performed with pads neatly trimmed as shown, it is expected that they would need replacement on a frequent basis, albeit less frequently than the misaligned pads of FIGS. 3A and 3B.